Available as one of methods for manufacturing a welded steel pipe is a process for manufacturing a steel pipe product by continuously supplying a steel strip and forming the steel strip into a cylindrical shape by abutting and welding both edges of the steel strip together. Electric resistance welding (an ERW process) was very often used as a welding method in the abovementioned manufacturing process. Lately, however, laser beam welding that uses a laser beam as a heat source has come to be used more often. The development of laser beam welding machines having an oscillation wavelength shorter than that of the conventional gas-oscillating laser (gas laser), such as a semiconductor laser and a fiber laser, allows occurrence of reduction in efficiency to be reduced, the reduction in efficiency occurring due to, for example, generation of plasma as a result of an interaction between a vaporized metal workpiece to be welded and the laser in the laser beam welding method. This results in an increasingly widespread use of the laser beam welding method.
The laser beam welding method requires accurate positioning to ensure that an irradiation point of the laser beam falls on the abutting portions at all times. In the continuous steel strip forming process, however, the abutting position of the steel strip tends to fluctuate due to, for example, conditions of a manufacturing line and a heat input. To accurately control the irradiation point of the laser beam, therefore, techniques have been researched and developed for continuously detecting the irradiation point of the laser beam at the laser beam welding portion.
One known method for detecting the welding position in the laser beam welding incorporates a television camera that directly observes the welding portion (near the laser beam irradiation point) to thereby detect a central position of the abutting portions and a molten pool (see Patent Literature 1). In this welding position detecting method, the position of external illumination is designed such that the welding portion is irradiated with illumination from the external illumination so that the molten pool is brightly observed, while the abutting portions are dimly observed. A luminance pattern of the observed image is then binarized to thereby detect the central position of the abutting portions and the molten pool.
Another known method for detecting the welding position in the laser beam welding incorporates an imaging device that images light reflected from the welding portion (near the laser beam irradiation point) irradiated with the laser beam and light generated from a plasma and determines, out of the captured image, a portion having luminance higher than that of surrounding areas to be the laser beam irradiation point (see Patent Literature 2).